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Page:Popular Science Monthly Volume 19.djvu/456

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THE POPULAR SCIENCE MONTHLY.

under the front edge of the dorsal fin, or even in front of it; while in the herring, though the position of the ventral fin varies a little, it lies more or less behind the front margin of the dorsal fin. The anal fin is of the same length as the dorsal, in the herring, longer than the dorsal in the sprat. But the best marks of distinction are the absence of vomerine teeth in the sprat, and the smaller number of pyloric cæca, which do not exceed nine, their openings being disposed in a single longitudinal series.

Shads and pilchards have a common character by which they are very easily distinguished from both sprat and herring. There is an horizontal fold of scaly skin on each side of the tail above and below the middle line. Moreover, they have no teeth in the inside of the mouth, and their pyloric cæca are very numerous—a hundred or more—their openings being disposed five or six in a row.

The shads have a deep narrow notch in the middle line of the upper jaw, which is absent in the pilchard. The intestine of the shad is short and straight, like that of the herring; while that of the pilchard is long, and folded several times upon itself.

Both of these fishes, again, possess a very curious structure, termed an accessory branchial organ, which is found more highly developed in other fishes of the herring family, and attains its greatest development in a fresh-water fish, the Heterotis, which inhabits the Nile. This organ is very rudimentary in the shad (in which it was discovered by Gegenbaur[1]), but it is much larger in the pilchard, in which, so far as I know, it has not heretofore been noticed. In Chanos and several other Clupeoid fishes it becomes coiled upon itself, and in Heterotis the coiled organ makes many turns. The organ is commonly supposed to be respiratory in function; but this is very doubtful.

Herrings which have attained maturity, and are distended by the greatly enlarged milt or roe, are ready to shed the contents of these organs, or, as it is said, to spawn. In 1862 we found a great diversity of opinion prevailed as to the time at which this operation takes place, and we took a great deal of trouble to settle the question, with the result which is thus stated in our report:

We have obtained a very large body of valuable evidence on this subject, derived partly from the examination of fishermen and of others conversant with the herring-fishery; partly from the inspection of the accurate records kept by the fishery officers at different stations, and partly from other sources; and our clear conclusion from all this evidence is, that the herring spawns at two seasons of the year, in the spring and in the autumn. We have hitherto met with no case of full or spawning herrings being found, in any locality, during what may be termed the solstitial months, namely, June and December; and it would appear that such herrings are never (or very rarely) taken in

  1. "Ueber das Kopfskelet von Alepocephalus rostratus" ("Morphologisches Jahrbuch," Bd. iv., Suppl., 1878).